The present invention relates to an impression roller for an electrostatically assisted printing machine.
Printing machines have been developed in which the transfer of ink to a web of material, such as paper, is not only provided by direct transfer of ink from an engraved cylinder, but also by electrostatic attraction of ink to the web. This is accomplished by passing the web through a nip region where the underside of the web is contacted by the engraved cylinder as it is being pressed down against the cylinder by an impression roller, which in turn is urged downward by hydraulic means. Transfer of the ink is assisted by applying an electrical voltage in the nip region in a pattern determined by the engraved cylinder. In machines where a backup roller contacts the impression roller in an area halfway around its circumference from the nip region, a voltage may be conveniently applied to the impression roller at that point and conducted through it to the engraved cylinder which is at ground potential.
Impression rollers of the type suitable for use in such a machine have typically been formed around a steel core that is grounded and insulated from the semiconductive layer that is formed on the exterior of the roller to conduct current to the nip region. The material used in the outer layer of the impression roller exhibits the resistance of a semiconductor, and is also resilient, which allows for flattening of the outer surface of the impression roller in the nip region.